Optical recording media such as optical discs are becoming more and more multi-layered. As a result, the correction of spherical aberration caused by the difference in depth between layers read by an optical head (an optical pickup) has become a problem. In a conventional method of correcting such spherical aberration by the mechanical drive of a lens, the optical head is large-sized due to the mechanism for driving the lens. JP Patent Publication (Kokai) No. 2005-235326 A and JP Patent Publication (Kokai) No. 2002-357804 A, for example, propose optical heads in which the mechanism for driving the lens is omitted by electrically switching lens actions using a liquid crystal lens element in place of the mechanical drive, thereby reducing the size of the optical head.
JP Patent Publication (Kokai) Nos. 10-97720 A (1998), 11-25467 A (1999), and 2005-122862 A disclose objective lens position sensors. JP Patent Publication (Kokai) No. 9-115146 A (1997) discloses that a position sensor and a speed sensor are mounted on a focus actuator. However, none of these publications disclose the concrete structure of the sensors.
(Patent Document 1) JP Patent Publication (Kokai) No. 2005-235326 A
(Patent Document 2) JP Patent Publication (Kokai) No. 2002-357804 A
(Patent Document 3) JP Patent Publication (Kokai) No. 10-97720 A (1998)
(Patent Document 4) JP Patent Publication (Kokai) No. 11-25467 A (1999)
(Patent Document 5) JP Patent Publication (Kokai) No. 2005-122862 A
(Patent Document 6) JP Patent Publication (Kokai) No. 9-115146 A (1997)